EPIDs (Electronic Portal Imaging Device) are widely used with linear accelerators (linacs) for patient setup verification and records. Such an imaging device sits opposite the linear accelerator, in line with the radiation beam, such that in use the patient is positioned between the two. The EPID therefore detects the (therapeutic) radiation after it has passed through the patient, and the image produced illustrates the extent of the radiation beam and its alignment with a treatment region (e.g., a tumour) within the patient.
It is common for the radiation beam to be rotated around the patient during a course of radiotherapy, allowing the therapeutic radiation to be directed towards the target region in the patient from a number of different directions. By keeping the target region at or close to the isocentre of the system, collateral damage to tissue surrounding the target can be minimized. To achieve this, both the radiation source and the EPID tend to be mounted on opposite sides of a rotatable gantry.
EPIDs are usually mounted on an extension arm to allow the device to be extended into the path of the radiation beam during use, and retracted when not in use. A conventional arrangement is shown in FIGS. 1a and 1b, with the former showing the apparatus in its extended form and the latter showing the apparatus in its retracted form.
The apparatus has a mount 12 for connection to the gantry of the radiotherapy system, a detector 16 which folds outwards to lie transverse to the radiation beam, and an extending arm 14 connecting the detector 16 to the mount 12.
The extension arm 14 has a “scissors” structure, i.e. two members are pivotally linked to one another at their centres to form a pair of members, and a plurality of such pairs are linked to each other by pivotal connection at their respective ends. By opening or closing one pair of interconnected members, each pair in the arm is also opened or closed, resulting in extension or retraction of the arm as a whole. It can also be seen that the detector 16 unfolds as the arm 14 extends, although this is not crucial.
The scissors structure presents some problems, however. For example, the connections of the mount 12 and the detector 16 to respective ends of the extending arm 14 are difficult. To simplify the connecting mechanism in each case, the end of one cross-member is positionally fixed to the mount/detector, and the other cross-member is slidably connected to the mount/detector (both are able to pivot). In this way, the arm 14 is reliably connected at each end. However, the act of extending or retracting the arm then results in a raising or lowering of the centre of gravity of the arm and the detector 16. For example, although FIGS. 1a and 1b are schematic, it can be seen that the centre of gravity in the latter arrangement is higher (i.e. up the page) than in the former arrangement.
This situation is further complicated by the rotation of the gantry. At some angles of rotation, gravity will act on the arm and detector so as to force the arm to retract; at other (opposite) angles of rotation, gravity will act in the opposite direction, forcing the arm to extend. In intermediate angles of rotation, the centre of gravity may move laterally or in a direction having both lateral and vertical components.
What is needed is an extension mechanism which works reliably and with the same level of effort regardless of the angle of rotation of the gantry.